1. Field of the Invention
The present invention relates to a circuit pattern forming method and a circuit pattern forming device that are used to form circuit patterns by ejecting a conductive pattern forming solution and an insulating pattern forming solution onto a substrate.
2. Description of the Related Art
In forming patterns on printed circuit boards, a subtractive method has been in general use. The subtractive method, however, requires a large number of fabrication processes and its cost occupies a large percentage of an overall manufacturing cost. To deal with this problem, a so-called liquid ejection method, which requires a smaller number of steps and is suited for small-lot production of a wide variety of products, has been proposed in recent years. This liquid ejection method draws a pattern on a substrate by ejecting a pattern forming solution onto the substrate, and is disclosed in U.S. Patent Application Publication No. 2004/0000429 (corresponding to Japanese Patent Application Laid-open No. 2003-309369).
The '429 publication discloses a method of forming wiring patterns by moving a substrate and a liquid ejection head relative to each other, and causing the head to eject liquid droplets as the substrate and the head are moved. An overview of this pattern forming method will be explained by referring to FIG. 10A to FIG. 10K.
FIGS. 10A to 10K are cross-sectional views of a multilayered printed circuit board during the manufacturing process. FIG. 10A shows a drop of a conductive pattern forming solution, containing fine metal particles, being ejected from a liquid ejection head 101 onto a substrate 102. Here, designated by reference numeral 104, are dots formed by the ejected droplets 103 landing on the substrate 102. Landing intervals of the dots 104 are such that they do not contact each other. After the conductive pattern is formed, a solvent of the conductive pattern forming solution is dried, to a state shown at 105 of FIG. 10B. The dots 105 that are formed after the solvent is evaporated are thinner than before they were dried.
Next, as shown in FIG. 10C, dots 106 are formed between adjoining dots 105, by ejecting droplets of the conductive pattern forming solution onto positions between the previously formed dots. Then, they are dried by evaporating the solvent to form a pattern 107, with all its dots connected, as shown in FIG. 10D. Further, to form the conductive pattern to a desired thickness, the above ejection and drying steps are repeated to form first layer conductive pattern 108, as shown in FIG. 10E.
Next, on the already formed conductive patterns, the conductive pattern forming solution is ejected, to form an interlay conduction post 109, for electrically connecting the first and second layers (see FIG. 10F). Then, the patterns are heated to have metal particles in the interlayer conduction post 109 electrically contact one another, making the pattern 108 and the interlayer conduction post 109 an integral pattern (see FIG. 10G).
Then, as shown in FIG. 10H, a solution 110 to form an interlayer insulation film is applied to a thickness such that the interlayer conduction post 109 slightly protrudes from the insulation film. Then, the patterns are heated, to remove the solvent and to harden the insulating material, with the result that the thickness of the interlayer insulation film 110 is reduced to half, as shown in FIG. 10I. Here, the insulation pattern forming solution is applied again (FIG. 10J), and subjected to a heating step to form an almost flat insulation film, as shown in FIG. 10K.
In forming a second layer, the above pattern forming procedure is repetitively performed over the first layer of FIG. 10K, to form a multilayered circuit.
As described above, with the liquid ejection type circuit forming method that uses the liquid ejection head, the number of steps required is relatively small, and thus, the printed circuit board can be constructed inexpensively. However, when ejecting liquid droplets from nozzles of the liquid ejection head, small droplets, called satellites, are also sprayed with the main droplets, causing problems to the circuits being formed.
The above problem will be explained in detail, as well as a droplet ejection process of the head, and how the satellites are formed along with the main droplets.
FIGS. 11A to 11F are cross-sectional views showing how a circuit forming liquid (referred to simply as a liquid), such as a conductive pattern forming solution and an insulation pattern forming solution, is ejected from the liquid ejection head. In each figure, portions shown shaded represent the solution. As shown in FIG. 11A, the circuit forming solution is filled from a liquid chamber 121 through a liquid path 122 up to an opening of a nozzle 123. Denoted by 124 is a heater, which is formed in a silicon substrate 125. The solution present in the liquid chamber 121 advances into the liquid path 122 by a capillary attraction. At the same time, the solution in the liquid path 122 is acted upon by a negative pressure generated by a supply unit, such as a liquid tank, and tends to be drawn back into the liquid chamber 121. Therefore, when no solution ejection is performed, these two forces balance holding the solution, as shown. At this time, the solution in the nozzle 123 forms a concave meniscus 126, as shown by the negative pressure acting toward the liquid member 121.
FIGS. 11B to 11F show how a bubble is formed, a droplet is ejected and a meniscus is formed, as the actual ejection operation is performed, as opposed to the static state shown in FIG. 11A.
When a printing operation is started and a voltage is applied to the heater 124, thermal energy is generated by the heater 124, to heat the solution in the liquid path 122, producing a bubble 127, as a result of film boiling. The bubble 127 continues to expand, while the heater 124 is energized, and the expansion force displaces the solution in the liquid path 122. That is, the solution near the nozzle 123 breaks the meniscus 162 and protrudes out, and the solution present near the liquid chamber 121 moves toward the liquid chamber 121, as shown in FIG. 11B.
Further, when the voltage application to the heater 124 is stopped, with the solution greatly protruding from the nozzle 123, as shown in FIG. 11B, the bubble 127 contracts, withdrawing the solution near the nozzle 123 greatly into the liquid path 122. At this time, the solution that was protruding outside parts from the solution being withdrawn into the liquid path 122 and, as shown in FIG. 11C, flies in a direction of the arrow. The solution that was sent flying includes a main droplet 128 followed by smaller droplets, called satellites 129, both of which land on the substrate.
The meniscus 126, withdrawn into the liquid path 122 by the capillary attraction after the bubble has vanished, now moves toward the nozzle 123 again, filling the solution into the liquid path 122 (see FIG. 11D). During this refilling process, the solution meniscus changes its state from the one shown in FIG. 11C to the one shown in FIG. 11D, and further moves to an initial state near the nozzle. Because of inertia, the meniscus cannot stop at the initial state, but slightly bulges out from the nozzle 123 (FIG. 11E).
With the meniscus slightly bulging, the surface tension of the solution and the negative pressure in the tank combine to pull the meniscus into the nozzle 123. This oscillates the solution and the oscillation progressively attenuates until it finally stops (FIG. 11F).
One printing operation ejects one main droplet 128 from a nozzle, followed by one or more satellites 129. It is known that the size of the satellites 129 and their distances from the main droplet 128 vary from one nozzle 123 to another, and that the ejection performance also changes from one operation to another, even in one and the same nozzle.
In the circuit pattern forming method described above, there is a possibility that these satellites may cause problems to the circuit operation. How the satellites are formed will be explained in detail by referring to FIGS. 12A to 12C.
First, a relation between the landing positions of the main droplet and satellites ejected from a nozzle of the head during scanning will be explained, by referring to FIGS. 12A-12C. FIG. 12A and FIG. 12B illustrate the process of drawing a pattern by moving the ejection head and the substrate relative to each other as the head ejects droplets. FIG. 12C shows shapes of droplets formed on the substrate in one ejection operation. The relative movement (scan) is done by holding the substrate 132 immovable, and moving the head 131 from the left toward the right in the figure.
When a droplet 134 is ejected from the nozzle 133 of the head 131, since the head 131 is moving toward the right, the droplet 134 flies down diagonally toward the right, from the ejected position, as shown in FIG. 12A. Then, as shown in FIG. 12B, the main droplet 134 is accompanied by one or more satellites 135 as it is ejected. This satellite 135, too, falls down diagonally toward the right. The satellite 135, since it is formed following the main droplet 134, lands at a position slightly shifted to the right (in the scan direction) from the main droplet 134. Conversely, when the scan is executed in the opposite direction, i.e., toward the left, the satellite 135 lands at a position to the left of the main droplet 134 (not shown). A distance D between the main droplet 134 and the satellite 135 varies, depending on a head scan speed, a droplet ejection speed, a droplet volume, components of a droplet, and a distance from the nozzle to the substrate.
Next, the process of drawing a pattern by continuously ejecting droplets will be described. FIG. 13A to FIG. 13C show the pattern drawing process, FIGS. 13A and 13B being cross-sectional views, and FIG. 13C, a plan view. It is assumed that the ejection head scans from the left to the right.
FIG. 13A shows a pattern 140 drawn on the substrate 132, with a satellite 141 accompanying the ejected droplets shown to have landed near the pattern 140. FIG. 13B shows the pattern thickened by applying droplets to the same positions again. Although a satellite 142 is also formed at this time, since the distance between the landing positions of the main droplet and the satellite often varies from one ejection operation to another, as described above, the satellite 142 that has landed in the subsequent scan often does not land on the preceding satellite 141. When the droplet application is repeated until the pattern reaches a desired thickness, there are cases in which satellites may contact one another, as shown enclosed by a one-dot line in the plan view of FIG. 13C, depending on the landing state of the satellites. In these cases, two patterns may get connected or short-circuited, resulting in an abnormal circuit operation.